<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <input type="text" name="" id="num">
    <script>
        var i = parseInt(0),
            sub = parseFloat(0);

        for (var i = 1; i <= 100; i++) {
            if (i % 2 !== 0) {
                sub += 1.0 / i; //sub=sub+1.0/i
            } else {
                sub -= 1.0 / i; //sub=sub-1.0/i
            }

        }
        document.getElementById("num").value = sub;
    </script>
</body>

</html>